JP6498309B2 - Scanning transmission electron microscope equipped with an electron beam energy loss spectrometer and its observation method - Google Patents

Description

The present invention relates to a scanning transmission electron microscope equipped with an electron beam energy loss spectrometer.

  Non-Patent Document 1 describes a method of combining a scanning transmission electron microscope (STEM) with an electron energy loss spectroscopy (EELS).

  STEM is an apparatus that observes the structure of a sample with high spatial resolution using an electron beam. Moreover, EELS can acquire the energy loss spectrum by interaction with a sample with high energy resolution by using the energy spectroscope attached as an accessory of STEM. Furthermore, an energy filter image can be obtained by selectively detecting electrons having a specific energy.

  When an electron beam is irradiated onto a thin film sample, it interacts according to the type and structure of the elements that make up the sample. Various information can be obtained by selectively detecting the angle and energy of the transmitted electron beam.

  For example, an image formed by electrons scattered at a low angle of several tens of mrad or less or transmitted without scattering is called a bright field image. On the other hand, the electron beam scattered at a high angle includes information depending on the density of the sample and is suitable for identifying the constituent elements, and is called a dark field image. When a dark field image is obtained by an annular detector, an optimum value exists in the scattering angle range to be detected. Although it depends on the acceleration voltage, for example, at 200 kV, it is desirable to set appropriately within a range of about 20 mrad to 300 mrad.

  Similarly, there is an optimum value for the scattering angle detected by EELS. In Non-Patent Document 2 (Section 61), inelastically scattered electrons accompanying plasmon excitation spread outside the center beam, so that the detection efficiency increases as the scattering angle detected increases, and the S / N is good. It is described that the analysis result is given.

  In Patent Document 1, in a TEM / STEM apparatus equipped with EELS, an electron lens is arranged between an annular dark field electron detector and a bright field electron detector, and an object point of the EELS spectrometer is made a virtual image. Thus, it is described that the mechanical incident angle to the EELS spectrometer is reduced without changing the angle of incorporation into the annular dark field electron detector and the angle of incorporation into the bright field electron detector. .

JP 2004-319233 A

R.F. Egerton: Electron Energy-Loss Spectroscopy in the Electron Microscope, Third Edition, Plenum Press Daisuke Shindo, Tetsuo Oikawa: Analytical electron microscopy for material evaluation, Kyoritsu Shuppan

  The inventor of the present application diligently studied high-resolution observation of bright-field STEM, dark-field image STEM, and EELS at a low acceleration voltage of 40 kV or less for the purpose of avoiding sample damage due to the primary electron beam and enhancing contrast. As a result, the following knowledge was obtained.

  In the following description, the term “take angle” refers to the scattering angle converted on the sample surface detected by the detector. When referring to the angle seen from the electron beam incident on the detector, it is expressed as “incident angle”.

  The bright-field STEM, dark-field image STEM, and EELS have different appropriate capture angles, and it is desirable to adjust appropriately according to the observation conditions.

  Non-Patent Document 1 (Section 103) describes a method of using a lens arranged on the downstream side of a sample as a method of controlling the angle of capture.

  However, when using a lens arranged on the downstream side of the sample, it is possible to focus an electron beam extending outward from the central beam and control the angle of capture. It must be placed away from the sample. In this case, chromatic aberration inevitably increases with the control of the capture angle, leading to deterioration of the energy resolution of EELS. In particular, at a low acceleration voltage, the influence of chromatic aberration is relatively likely to occur compared to a high acceleration voltage.

  As described in Non-Patent Document 2, the detection efficiency of EELS increases as the scattering angle to be detected increases. However, when the detection angle is increased randomly, the energy resolution of EELS deteriorates due to the aberration of the energy spectrometer. May end up. For this reason, it is desirable to adjust appropriately according to observation conditions. Further, even when the scattering angle that can be taken into the detector is the same, the detection efficiency is deteriorated as the acceleration voltage is lower.

  Further, in Patent Document 1, due to space limitations, the electron lens must be disposed at a position away from the sample, so that the focal length becomes long and chromatic aberration inevitably becomes a problem. Since Patent Document 1 is a TEM / STEM and assumes a high acceleration voltage, it does not pay attention to the problem of chromatic aberration.

An object of the present invention relates to high-resolution observation of a bright field STEM, a dark field image STEM, and an EELS at a low acceleration voltage.

The present invention relates to a transmission scanning electron microscope equipped with an electron beam energy loss spectrometer, and by changing the arrangement of the sample with respect to the optical axis direction of the primary electron beam, the capture angle of the STEM detector and the electron beam energy loss spectrometer Related to controlling.

According to the present invention, it is possible to easily control the optimum scattering angle for each of the bright field STEM, dark field STEM, and EELS while suppressing the occurrence of chromatic aberration associated with the control of the capture angle.

Schematic configuration diagram of STEM with EELS according to Example 1 Schematic explaining the focusing action of the objective rear magnetic lens Graph showing the relationship between the position of the sample and the magnification of the objective rear magnetic lens 1 is a schematic side view of a stage drive mechanism according to a first embodiment. Sectional drawing of the principal part of the sample holder front-end | tip concerning Example 2. Sectional drawing of the sample stand with various height concerning Example 2.

  In the embodiment, an electron source that emits a primary electron beam, a stage driving mechanism that moves a sample stage that holds a sample, an objective magnetic field lens that focuses the primary electron beam on the sample, and a primary electron irradiated on the sample Transmission scanning type comprising a scanning coil that scans a line two-dimensionally, an STEM detector that detects electrons transmitted through the sample, and an electron beam energy loss spectrometer that detects an energy loss spectrum of electrons transmitted through the sample Disclosed is an electron microscope that controls the capture angle of a STEM detector and an electron beam energy loss spectrometer by changing the arrangement of a sample with respect to the optical axis direction of a primary electron beam.

  In addition, the Examples disclose a transmission scanning electron microscope in which the acceleration voltage for bright field STEM observation, dark field STEM observation, and EELS observation is 40 kV or less.

  In addition, the embodiment discloses a transmission scanning electron microscope that changes the arrangement of the sample with respect to the optical axis direction of the primary electron beam in accordance with switching between bright field STEM observation, dark field STEM observation, and EELS observation. In addition, a transmission scanning electron microscope that automatically changes control of the magnetic lens and the scanning coil in accordance with switching is disclosed.

  In addition, the embodiment discloses a transmission scanning electron microscope that adjusts the arrangement of the sample with respect to the optical axis direction of the primary electron beam by driving a stage driving mechanism.

  In addition, the embodiment discloses a transmission scanning electron microscope that adjusts the arrangement of the sample with respect to the optical axis direction of the primary electron beam by exchanging the sample table with different heights.

  Also, in the examples, STEM and EELS observation methods in a transmission scanning electron microscope equipped with an electron beam energy loss spectrometer, the arrangement of the sample with respect to the optical axis direction of the primary electron beam emitted from the electron source is changed. To control the angle of capture of an STEM detector and an electron beam energy loss spectrometer.

  Also, in the examples, an STEM and EELS observation method is disclosed in which the acceleration voltage of bright field STEM observation, dark field STEM observation, and EELS observation is 40 kV or less.

  Also, in the examples, an STEM and EELS observation method is disclosed in which the arrangement of the sample with respect to the optical axis direction of the primary electron beam is changed according to switching between bright field STEM observation, dark field STEM observation, and EELS observation. Also, STEM and EELS observation methods that automatically change the control of the magnetic field lens that focuses the primary electron beam on the sample and the scanning coil that scans the primary electron beam irradiated onto the sample in two dimensions according to the switching. Is disclosed.

  Also, in the embodiment, an STEM and EELS observation method is disclosed in which the arrangement of the sample with respect to the optical axis direction of the primary electron beam is adjusted by controlling a stage driving mechanism that moves the sample stage that holds the sample.

  Further, in the examples, an STEM and EELS observation method is disclosed in which the arrangement of the sample with respect to the optical axis direction of the primary electron beam is adjusted by exchanging the sample table with a different height.

The above and other novel features and effects will be described below with reference to the drawings.

  FIG. 1 is a schematic configuration diagram of a STEM provided with an EELS according to the first embodiment. The primary electron beam 19 generated by the electron source 1 is focused on the sample by the focusing lens 3 and the objective front magnetic lens 7. The primary electron beam 19 is scanned on the sample surface when a scanning signal is supplied from the electron optical control signal generator 22 to the electron beam scanning coil 5. The sample 30 is disposed in the magnetic field of the objective lens. Among the objective lenses, the objective front magnetic lens 7 is above the sample 30 and the objective rear magnetic lens 9 is below the sample 30. When the sample 30 is irradiated with the primary electron beam 19, secondary electrons 6 are generated and detected by the secondary electron detector 20 disposed above the objective front magnetic lens 7. Further, when the sample 30 is a thin film or fine particles and the acceleration voltage of the primary electron beam 19 is sufficiently high, the scattered electrons 10 pass through the sample 30. The scattered electrons 10 are detected by a dark field STEM detector 11, a bright field STEM detector 13, or an EELS spectrum detector 18 disposed below the objective rear magnetic lens 9.

  An electron beam that has passed through an aperture provided in the dark field STEM detector 11 is incident on the bright field STEM detector 13. Usually, since the capture angle is larger than necessary, the capture angle is limited using the bright field STEM stop 12.

When the bright field STEM diaphragm 12 and the bright field STEM detector 13 are retracted out of the optical axis, an electron beam that has passed through an aperture provided in the dark field STEM detector 11 is incident on the energy spectrometer 17. Since the take-in angle is also larger than necessary, the take-in angle is limited by the EELS entrance stop 14. The multipole lens 15 has a role of focusing the electron beam on the spectrum detector 18, and the quadrupole lens 16 has a role of expanding or reducing the chromatic dispersion generated by the energy spectrometer 17. By separating each electron beam loss energy, an energy spectrum can be obtained.

  FIG. 2 is a schematic diagram for explaining the focusing action of the objective rear magnetic lens. FIG. 3 is a graph showing the relationship between the position of the sample and the magnification of the objective rear magnetic lens. The control of the take-in angle will be described with reference to FIGS. The capture angle in each detector is adjusted by the angle magnification of the objective rear magnetic lens 9. Here, the angle magnification Ma is defined by | Ma | = β / γ. In this embodiment, as shown in FIG. 2, β is taken in and γ is used as the incident angle. As shown in FIG. 3, the angle magnification varies depending on the arrangement of the sample 30 in the direction of the optical axis 34. The angle magnification of infinity means a state where the virtual object point 32 can be regarded as infinity, that is, a state where the scattered electrons 10 have a trajectory parallel to the optical axis 34. Depending on the structure of the objective lens, the scattered electrons may be focused a plurality of times. In such a case, the position of the sample where the angular magnification is infinite reaches a plurality of positions.

  FIG. 4 is a schematic side view of the stage driving mechanism according to the present embodiment. In the present embodiment, the stage drive mechanism 21 is used as a specific means for changing the arrangement of the sample 30 with respect to the optical axis 34. The tip of the sample holder 8 inserted into the objective lens is in contact with the fine movement tube 26. The fine movement tube 26 is supported by a fine movement tube receiver 27. The vertical movement of the Z fine movement stage 28 in the direction of the optical axis is transmitted to the support rod of the sample holder 8 and converted into a rotational movement. As a result, the position of the sample 30 in the optical axis direction changes, and the capture angles of the bright field STEM, dark field STEM, and EELS change. Since the appropriate take-in angle differs depending on the observation conditions, the arrangement of the sample is adjusted appropriately by stage driving. In this case, the operating range of the sample is about ± 0.3 mm.

  In the apparatus, a plurality of observation modes are registered in advance for each purpose. The user selects a mode on the screen on the monitor 23 as necessary. Then, a control signal is sent from the stage control signal generator 35 to the Z fine movement stage 28, and is automatically set to the position of the sample 30 suitable for the selected observation mode. At this time, at the timing when the observation mode is switched, a control signal is sent from the electron optical control signal generator 22 to the electron beam scanning coil 5, the focusing lens 3, and the objective lens, and is automatically reset to the optimum control value. . The observation modes are registered with names such as EELS high S / N mode and dark field STEM heavy element observation mode, for example.

One of the advantages of adjusting the angle magnification by the arrangement of the sample 30 with respect to the optical axis 34 is that an optical system with small chromatic aberration can be realized. Since the sample 30 is disposed in the magnetic field of the objective lens, the focal length is extremely small, and accordingly, chromatic aberration acting on the scattered electrons 10 can be suppressed. In order to prevent the sample 30 from being damaged by the irradiation of the primary electron beam 19 or to observe the contrast at a low acceleration voltage for the purpose of enhancing the contrast, the influence of chromatic aberration is relatively likely to occur as compared with a high acceleration voltage. The compatibility with the above configuration is very good.

  In this embodiment, the basic operation of the electron microscope is the same as that of the first embodiment, but the difference is that the exchange of the sample stage is used as means for changing the arrangement of the sample. Hereinafter, the difference from the first embodiment will be mainly described.

FIG. 5 is a cross-sectional view of the main part of the tip of the sample holder according to the present embodiment, and FIG. 6 is a cross-sectional view of the sample stage having various heights according to the present embodiment. The sample stage 29 is fixed to the tip of the sample holder 8 and has a structure having an opening through which the scattered electrons 10 pass. A sample is attached to the top of the opening. This sample stage is fixed to the tip end of the sample holder 8 so as to be removable by screws, push springs, adhesive paste or the like. As shown in FIG. 6, sample stands 29 a, 29 b, 29 c and the like having various heights are prepared, and the arrangement of the sample in the optical axis direction can be changed by exchanging the sample stands 29. In this case, since there is no particular limitation on the shape of the sample stage, the arrangement of the sample can be greatly changed as compared with the first embodiment.

In this embodiment, the first embodiment and the second embodiment are used in combination. In other words, in order to change the arrangement of the sample 30 in the optical axis direction, the driving of the Z fine movement stage 28 and the replacement of the sample stage 29 are used together, so that a more flexible response is possible.

The present invention can be applied to a TEM / STEM in which the acceleration voltage can be 100 kV or higher, but is particularly suitable for a scanning electron microscope having a maximum acceleration voltage of 40 kV or lower.

DESCRIPTION OF SYMBOLS 1 Electron source 3 Focusing lens 4 Objective diaphragm 5 Electron beam scanning coil 6 Secondary electron 7 Objective front magnetic lens 8 Sample holder 9 Objective rear magnetic lens 10 Scattered electron 11 Dark field STEM detector 12 Bright field STEM diaphragm 13 Bright field STEM detection 14 EELS entrance stop 15 multipole lens 16 quadrupole lens 17 energy spectrometer 18 EELS spectrum detector 19 primary electron beam 20 secondary electron detector 21 stage drive mechanism 22 electro-optic control signal generator 23 monitor 24 magnetic pole on objective lens 25 objective lens lower magnetic pole 26 fine moving tube 27 fine moving tube receiver 28 Z fine moving stage 29 sample base 29a sample base (high)
29b Sample stage (middle)
29c Sample stage (low)
30 Sample 31 Electron beam 32 Virtual object point 33 Aperture 34 Optical axis 35 Stage control signal generator

Claims (12)

An electron source emitting a primary electron beam;
A stage drive mechanism for moving a sample stage for holding a sample;
An objective magnetic lens for focusing the primary electron beam on the sample;
A scanning coil that two-dimensionally scans the primary electron beam irradiated on the sample;
A STEM detector that detects the electrons that have passed through the sample;
In a transmission scanning electron microscope comprising: an electron beam energy loss spectrometer that detects an energy loss spectrum of electrons transmitted through a sample;
A transmission scanning electron microscope characterized in that the capture angle of the STEM detector and the electron beam energy loss spectrometer is controlled by changing the arrangement of the sample with respect to the optical axis direction of the primary electron beam. The transmission scanning electron microscope according to claim 1,
A transmission scanning electron microscope characterized in that the acceleration voltage of bright field STEM observation, dark field STEM observation and EELS observation is 40 kV or less. The transmission scanning electron microscope according to claim 1,
A transmission scanning electron microscope characterized in that the arrangement of the sample with respect to the optical axis direction of the primary electron beam is changed according to switching between bright field STEM observation, dark field STEM observation, and EELS observation. The transmission scanning electron microscope according to claim 3,
A transmission scanning electron microscope characterized by automatically changing the control of the objective magnetic field lens and the scanning coil in accordance with the switching. The transmission scanning electron microscope according to claim 1,
A transmission scanning electron microscope characterized by adjusting the arrangement of the sample with respect to the optical axis direction of the primary electron beam by driving the stage driving mechanism. The transmission scanning electron microscope according to claim 1,
A transmission scanning electron microscope characterized by adjusting the arrangement of the sample with respect to the optical axis direction of the primary electron beam by exchanging with a sample stage having a different height. A put that observation method in transmission scanning electron microscope having an electron beam energy loss spectrometer,
An observation method characterized by controlling the capture angle of an STEM detector and an electron beam energy loss spectrometer by changing the arrangement of a sample with respect to the optical axis direction of a primary electron beam emitted from an electron source. A observation method according to claim 7,
An accelerating voltage for bright field STEM observation, dark field STEM observation and EELS observation is 40 kV or less. A observation method according to claim 7,
An observation method comprising changing the arrangement of the sample with respect to the optical axis direction of the primary electron beam in accordance with switching between bright field STEM observation, dark field STEM observation, and EELS observation. A observation method according to claim 9,
An observation method characterized by automatically changing control of a magnetic lens for focusing a primary electron beam on a sample and a scanning coil for two-dimensionally scanning the primary electron beam irradiated on the sample in accordance with the switching. A observation method according to claim 7,
An observation method, wherein the arrangement of the sample with respect to the optical axis direction of the primary electron beam is adjusted by controlling a stage driving mechanism for moving a sample stage for holding the sample. A observation method according to claim 7,
An observation method, wherein the arrangement of the sample with respect to the optical axis direction of the primary electron beam is adjusted by exchanging with a sample stage having a different height.

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